This invention pertains to a high resolution and sensitive magnetic microscope using one of a plurality of different magnetic sensor types, including a magnetic tunneling junction, a spin valve, and an extraordinary Hall effect sensor. Suitable applications for the magnetic microscope include semiconductor integrated circuit (IC) and data storage research and manufacturing monitoring, as well a number of biological, chemistry, physics, and materials research applications.
The need for a sensitive magnetic microscope with a high degree of spatial resolution is felt in many industries. For example, IC engineers and designers can use such a microscope to carry out non-invasive measurements of electrical current distributions within an IC chip, and they may survey the operation of tiny devicesxe2x80x94pinpointing electrical defects down to the smallest unit (transistor, diode, interconnects, etc.). They may also study electro-migration of interconnects to develop even finer conducting lines. In another application, a data-storage professional can image ultrafine domain structures of future recording media with a magnetic microscope of sufficient sensitivity and resolution. These media will have an increasingly small domain size, eventually approaching the adverse superparamagnetic limit. Furthermore, the magnetic microscope is also applicable to a range of basic research areas, such as flux line structures.
Many physical objects generate magnetic fields (H) near the objects"" surfaces, and the magnetic microscope can obtain images of the magnetic fields by scanning a magnetic sensor on the surface of the object of interest. Such images can be spatially microscopic and weak in field strength. Nevertheless, these images reveal important signatures of inherent electrical and magnetic processes within the objects. For example, the magnetic image of a magnetic thin film discloses its internal magnetic domain structure (spatial electron-spin configuration); The electrical currents within an integrated circuit (IC) chip generate external magnetic images, which not only contain information of current-distribution, but also the frequencies with which various components on a chip are operating; A type II superconductor also creates an image of threading magnetic flux lines, whose structure and dynamics are fundamental properties. Researchers have confirmed the d-wave symmetry in high-Tc superconductors by studying the flux line images in a uniquely designed sample.
There are currently a number of techniques for imaging magnetic fields at surfaces.
Electron holography and SEMPA (scanning electron microscopy with polarization analysis) require high vacuum operation and delicate sample preparation. Both techniques offer static field images with good spatial resolution. However, these instruments are expensive and demand great technical skill to operate.
The magneto-optical microscope is a relatively simple system and is suitable for time-resolved imaging. However its field sensitivity and spatial resolution are poor.
The conventional scanning magnetic microscope has a microscopic field sensor, typically a superconducting quantum interference device (SQUID), a Hall probe, or simply a magnetic tip. This type of microscope scans the magnetic sensor relative to a sample to obtain a local field image. Though very sensitive, a SQUID probe is poor in resolution (xcx9c5 xcexcm), and requires cryogenics (77K). A Hall probe can operate under ambient conditions, but its sensitivity is low. The magnetic microscope equipped with a magnetic tip can only measure the gradient of the magnetic field, and cannot sense a high frequency signal (e.g. MHz-GHz).
Prior to this invention the conventional magnetic imaging systems, such as the conventional scanning magnetic microscope, suffered from at least one of an inadequate sensitivity or an inadequate spatial resolution, resulting in the generation of magnetic field images that were less than optimum.
An aspect of this invention lies in the adaptation of high-performance magnetic sensors, for example, the magnetic tunneling junction (MTJ), as the sensing element for a magnetic microscope. MTJ sensors are superior to existing sensors. The microscope equipped with such a sensor offers excellent field-sensitivity and spatial resolution, and affords new abilities in the design and characterization of modern semiconductor chips and data storage media. The magnetic microscope operates over a wide signal frequency range, at room temperature and ambient pressure, offering cost-effective turnkey convenience
These teachings provide in one aspect a scanning magnetic microscope with a magnetic-tunneling-junction (MTJ) sensor. This is a type of magnetoresistive sensor with both a high field-sensitivity and spatial resolution. The microscope operates under ambient conditions. Other types of magnetic sensors, such as spin valves and extraordinary Hall effect sensors, can also be used.
The scanning magnetic microscope in accordance with these teachings may be equipped with one of three types of magnetic sensors: magnetic tunneling, spin valve, or extraordinary Hall sensor. The scanning magnetic microscope measures a spatial magnetic field generated on the surface of a sample, such as an electrical circuit or a magnetic data storage media, and converts the resulting magnetic field map into an electrical current density map. The scanning magnetic microscope may be used to obtain the electrical current distribution in an integrated circuit network, or in a planar magnetic structure, without physically contacting the surface.
The scanning magnetic microscope may be used to measure the electromigration of fine electrical interconnects. As the width of the interconnects becomes finer (state-of-the-art, 0.18 micron, next generation, 0.15 micron), the electrical current density in the interconnects can displace atoms away from their equilibrium positions and eventually cause the interconnects to break. With the scanning magnetic microscope one may observe how the interconnects are broken gradually under various types of conditions (e.g. elevated temperature, aging, material composition, overlay, edge roughness, etc.). Using the scanning magnetic microscope one may observe the electrical current distribution of interconnects that are embedded within dielectrics. Other microscope types, such as the scanning electron microscope, cannot see embedded interconnects. An important, but not limiting, application of the scanning magnetic microscope is thus in inspecting integrated circuits, as well as in investigating the electromigration mechanism.
The conventional scanning SQUID (superconducting quantum interference device) microscope operates at low temperature (77K) and has poor spatial resolution to image room temperature objects. The scanning magnetic microscope in accordance with these teachings operates under ambient conditions, and may achieve a spatial resolution as small as 0.03 microns.
One preferred embodiment of a magnetic tunnel junction (MTJ) sensor has the following layer structure: Si wafer or Si wafer coated with SiO2/60 Ta/300 Al/40 Ni79Fe21/x Cu(x: 0-100)/100 PtMn/(P2) Co/7Ru/(P1) Co/y Al2O3 (y: 3-20)/10 Co/70 Ni79Fe21/250 Al/75 Ta. The Cu layer is used to improve the growth of the PtMn layer exchange bias. An improved exchange bias enhances the magnetic stability of the sensor. The 10 Angstrom Co layer is used to improve magnetic stability, thereby reducing magnetic field noise. The magnetization of the pinned layers and the free layers are perpendicular to one another. The (P2)Co/7Ru/(P1)Co sequence is used to compensate the Neel coupling field, so that the magnetic offset field is zero in the sensor. A zero offset field increases the field dynamic range of the sensor.
A presently preferred magnetic tunneling junction (MTJ) multilayer structure has the form of Si wafer/SiO2/Pt (30 nm)/Py(x)/FeMn (13 nm)/Py (6 nm)/Al2O3 (0.5-2.5 nm)/Py (12 nm)/Al (49 nm). The seed layer Py (x) is either Fe79Ni21 (permalloy) with a thickness x between 2 to 10 nm, or Cu (1.5 nm)/Py (x) with x between 2 to 9 nm. The buffer layer Pt (30 nm) can be replaced by Ta (5 to 10 nm). With this layer sequence a magnetoresistance as large as 34% has been observed. Transmission-electron-micrographs of the cross section of the MTJ multilayer structure show high quality smooth and pin-hole free layer structures.
The extraordinary Hall sensor is a four terminal sensor, with two terminals for electrical current, and two terminals for measuring the Hall voltage, which is proportional to external magnetic field to be measured. The preferred composition of the extraordinary Hall sensor is FexPt100-x, or CoxPt100-x, and other associated alloys.
Magnetic images may be obtained in either a contact mode (sensor contacting surface) or in non-contact mode. The magnetic field spatial profile may be employed to obtain the height between the sensor and the surface of a sample. This height information is useful in calculating the magnitude of the current density.
A scanning magnetic microscope includes a specimen stage for holding a specimen to be examined; a sensor for sensing a magnetic field generated by the specimen, the sensor including one of a magnetic tunneling junction (MTJ) sensor, a spin valve sensor, or an extraordinary Hall effect sensor; translation apparatus for translating the sensor relative to a surface of said specimen; and a data processor, having an input coupled to an output of said sensor, for constructing an image of said magnetic field. In another embodiment a read/write head from a hard disk drive is shown to make a suitable magnetic sensor. The scanning magnetic microscope can be used for examining the current flow in integrated circuits and related phenomenon, such as electromigration, as well as magnetic data storage media and biomagnetic systems, to mention a few suitable applications.